Various publications, including patents, published applications, technical articles, and scholarly articles are cited throughout the specification. Each of these materials is incorporated by reference herein, in its entirety and for all purposes.
Industrial slings are typically prepared from metals or synthetic materials. Wire rope slings are commonly made of a plurality of metal strands twisted together and secured by large metal sleeves or collars. Synthetic slings are usually comprised of a lifting core made of twisted strands of synthetic fiber and an outer cover that protects the core. One popular design of synthetic slings is a roundsling in which the lifting core forms a continuous loop and the sling has a circular or oval-shaped appearance.
Modern industrial slings may experience failure and loss of a load caused by the sling breaking or failing, for example, because it was fatigued or because it was over-stretched or overloaded during a current or previous use. When subjected to an overload condition in excess of its rated capacity, a roundsling may be permanently damaged/deformed if the load stretches the fibers of the load bearing core material beyond their yield point. When a synthetic fiber sling is overloaded beyond its tensile strength or weight-lifting capacity at maximum stretch, it is considered to be fatigued and may never return to its normal strength and load bearing capacity.
Slings are generally provided with their load capacity (rated capacity) specified, particular sling. This allows users to know the lifting or load capacity of the sling. Nevertheless, this capacity is sometimes exceeded, either accidentally or by users engaging unsafe shortcuts during rigging and use of the sling. Often, over-load, fatigue, or damage to the sling materials may not be readily apparent, particularly given the large size or length of the sling, or because the load-bearing core is hidden inside the outer cover. If a roundsling has been fatigued or structurally changed, the sling may no longer be able to lift a load according to its maximum rated load capacity. Such a condition may potentially become a serious threat to the operators and riggers using the damaged sling.
Many commercially produced roundslings include a pre-failure indicator. Such failure indicators in practice have shown to be inconsistent, even among the same types of slings from the same manufacturer. Accordingly, there is a need in the art for more consistent and more reliable sling pre-failure indicators.